If the spherical aberration of the electron lenses in an electron gun is to be improved, the intensity of the electric field of the main lens has to be weakened, and at the same time, the diameter thereof has to be expanded. However, the diameter of the lens is restricted by the diameter of the neck of the cathode ray tube, and therefore, the range of improvement in the spherical aberration achievable through an increase of the diameter of the lens is very limited.
At present, therefore, a main lens consisting of a plurality of electron lenses generating relatively weak electric field intensities that overlap one another are used. Such a main lens means is disclosed in U.S. Pat. No. 4,253,041 and illustrated in FIG. 1, and constitutes a part of a multistep focusing type electron gun.
In FIG. 2, another type of electron gun, which is disclosed in U.S. Pat. No. 3,772,554, is shown. In that gun, the diameter of the main lens, constituting the final accelerating and focusing portion of the main lens, may be increased by providing a concave recess where beam passing holes are formed. This arrangement achieves the same effect as where a plurality of electron lenses are overlapped, as in the multistep focusing type electron gun described above. However, such an electron gun presents manufacturing difficulties because of the complicated press die required to make the beam passing holes.